The present application claims priority to Japanese Application No. P11-369266 filed Dec. 27, 1999, which application is incorporated herein by reference to the extent permitted by law.
This invention relates to a non-aqueous electrolytic cell comprising a positive electrode, a negative electrode and an electrolyte.
In recent years, a diversity of electronic appliances and devices have appeared, with their miniaturization, cordless tendency and weight saving being in progress. This, in turn, requires a higher capacity and more weight saving with respect to cells, particularly, those cells using non-aqueous electrolytes, for use as a power supply for driving the electronic devices.
Cells making use of dopingxe2x80xa2de-doping of lithium, e.g. so-called lithium ion secondary cells, ensure an energy density larger than lead storage batteries or nickel-cadmium batteries that are conventional secondary cells using aqueous electrolytic solutions. Accordingly, many studies and developments of lithium ion secondary cells have been extensively made at present.
Recently, lithium ion secondary cells have wide utility as a power supply for driving electronic appliances employed not only in a normal temperature environment, but also in a high temperature environment. This places importance on the stability of the cell employed in a high temperature environment.
In order to assure the cell stability in a high temperature environment, there have been proposed a non-aqueous electrolytic secondary cell wherein a positive electrode active material is so defined with respect to its average particle size that the area of contact between an electrolytic solution and the positive electrode active material decreases (Japanese Patent Laid-open No. Hei 9-283144) and a non-aqueous electrolytic secondary cell wherein part of constituent elements of a positive electrode active material is replaced by a different type of element (Japanese Patent Laid-open No. Hei 11-7958).
However, these cells are disadvantageous in that since the specific character of the positive electrode active material per se is altered, then inherent cell characteristics may be degraded, and limitation is placed on an application only to a positive electrode active material capable of substituting part of a constituent element with a different type of element.
Moreover, there has been proposed a non-aqueous electrolytic secondary cell using a flame-retardant non-aqueous electrolytic solution (Japanese Patent Laid-open No. Hei 8-37025). For fabrication of larger-sized cells and extension of an application range, higher stability for the cell is demanded.
In the lithium ion secondary cells, lithium and transition metal composite oxides such as LiCoO2, LiNiO2 and the like are used as a positive electrode active material.
It is well known that the positive electrode active material containing a lithiumxe2x80xa2transition metal composite oxide decreases in stability in a state where the cell is charged so that lithium ions are withdrawn and that when the electrode active material is heated in this state, the material releases active oxygen (see, for example, Solid State Physics, 69, 265 (1994)).
This active oxygen combines with an electrolyte to form an active peroxide intermediate, thereby decomposing the electrolyte in chain reaction. Eventually, heat is generated inside the cell.
Thus, the non-aqueous electrolyte secondary cell using these positive electrode active material has the problem on the stability when charged and discharged under high temperature environmental conditions.
It is accordingly an object of the invention to provide a non-aqueous electrolytic cell which overcomes the prior-art problems.
It is another object of the invention to provide a non-aqueous electrolytic secondary cell which exhibits a high capacity and good stability when charged and discharged under high temperature environmental conditions.
The above objects can be achieved, according to the present invention, by a non-aqueous electrolytic cell, which comprises a positive electrode having a positive-electrode-mix layer containing, at least, a positive electrode active material, a negative electrode having a negative-electrode-mix layer containing, at least, a negative electrode active material, and a non-aqueous electrolyte wherein a sulfur compound is added to at least one of the positive-electrode-mix layer, the negative-electrode-mix layer and the non-aqueous electrolyte.
The non-aqueous electrolytic cell of the invention having such an arrangement as described above should have a sulfur compound added to at least one of the positive-electrode-mix layer, the negative-electrode-mix layer and the non-aqueous electrolyte wherein the sulfur compound is able to decompose a peroxide intermediate formed during the course of charge and discharge in a high temperature environment. Thus, the electrolyte is suppressed from being decomposed.